The present invention is directed generally to the recording and retrieval of digital information on magnetic tape, and more particularly to methods and procedures for minimizing errors occurring during data transfer operations.
Conventional tape drive data storage apparatus employ various error correction and recovery methods to detect and correct data errors which, if left unresolved, would compromise the integrity of information read from or written to the magnetic tape media. Events which can lead to data errors include defects on the media, debris between the tape head and the media, and other conditions that interfere with head/media data transfer operations.
Error correction and recovery may be thought of as two distinct operations that are employed at different stages of error processing. Error correction is conventionally implemented using error correction coding (ECC) techniques in which random host data to be placed on a tape medium is encoded in a well-defined structure by introducing data-dependent redundancy information. The presence of data errors is detected when the encoded structure is disturbed. The errors are corrected by making minimal alterations to reestablish the structure. ECC error correction is usually implemented "on-the-fly" as data is processed by the tape drive apparatus. The well-known Reed-Solomon code is one cyclic encoding scheme which has been proposed for ECC error correction. Other encoding schemes are also known in the art.
Error recovery occurs when ECC error correction is unable to correct data errors or when thresholds for allowable correction are exceeded. The error recovery process usually requires stopping the tape and reprocessing a data block in which an error was detected. Typical error recovery procedures include tape refresh operations wherein a tape is wound to its end and brought back to the error recovery point, tape backhitch or "shoeshine" operations wherein a tape is drawn back and forth across the tape head, backward tape read operations, tape tension adjustment operations and tape servo adjustment operations, to name a few.
In prior art error recovery systems, it is common to perform a preprogrammed sequence of error recovery procedures in response to data errors that result in error recovery. In many cases, however, one or more error recovery procedures may not be required. For example, track fallout errors caused by localized tape defects or track fading due to debris adhering to either the tape media or the read/write heads can affect tracks for long stretches of tape as debris is dragged along. Errors of this type can often be resolved by reversing tape motion and dislodging the debris. In that case, other error recovery procedures may be unnecessary.
Often, it is possible to correct for track fallout using ECC error correction. ECC track fallout correction usually works well during data read operations. During data write operations, however, there is an increased risk that a subsequent read of the data will result in data transfer problems. With one or more of the tracks already lost as a result of write track fallout, a further loss of track data due to debris, edge damage and other media defects during read operations may cause error recovery to be invoked.
Accordingly, there is a need in the art for a system and method for recording and retrieving digital information on a tape wherein potential error recovery conditions are anticipated and resolved using minimal error recovery techniques. Rather than wait until persistent track fallout leads to the invocation of full error recovery, it would be preferable to address the problem in advance in a manner that does not require a complete complement of preprogrammed prior recovery procedures.